This invention relates to a security feature having a luminescent component and a component camouflaging the luminescent component.
The designation “value document” is to be understood within the framework of the invention to refer to bank notes, checks, shares, tokens, identity cards, credit cards, passports and also other documents as well as labels, seals, packages or other elements for product authentication.
Securing value documents against forgery by means of security features having a luminescent component has been known for a long time. The luminescent component is formed here by substances which will hereinafter also be designated luminophores and are formed by host lattices doped with transition metals or rare earth metals as luminescent ions. Such ions have the advantage that, after being suitably excited, they show one or more characteristic narrow-band luminescences which facilitate a reliable detection and the delimitation over other spectra. For doping, combinations of transition metals and/or rare earth metals have also been discussed. Such substances have the advantage that, in addition to the above-mentioned luminescences, one observes so-called energy transfer processes, which can lead to more complicated emission spectra. In such energy transfer processes, one ion can transfer its energy to another ion and the spectra can then consist of several narrow-band lines which are characteristic of both ions.
The stated security features for securing value documents have as a luminescent component individual luminophores whose emissions differ with regard to their spectral and/or temporal properties. The security features are incorporated into and/or applied to value documents in different forms of use. There can also be employed for the luminescent component a combination of luminophores. The emission bands of the employed luminophores constitute a spectral coding. Several different luminophores can be combined into systems, with the individual systems being independent of each other. The emission of the employed luminophores is also designated luminescence, whereby this may involve fluorescence and/or phosphorescence.
It is also known that the described security features are not formed solely by the luminescent component. As a further component, some security elements have a component that is used for camouflaging the luminescent component. For example, DE 30 48 734 A1 describes a security paper having camouflage substances protecting the authentication features. The camouflage substances of the camouflaging components correspond here substantially to the luminescent components, i.e. very similar or like-kind host lattices and dopants are employed for both the luminescent component and the camouflaging component. However, when manufacturing the camouflage substances for the camouflaging component it is made sure that the camouflage substances have no luminescent properties. For this purpose, parameters in the annealing or grinding process are for example changed for the camouflaging component in contrast to the manufacture of the luminescent component. Alternatively, so-called luminescence killers are employed. This prevents the luminescent component from being distinguished from the camouflaging component using conventional methods of analysis technology. By this means, primarily the position of the luminescent component is concealed, since it cannot be distinguished from the camouflaging component using conventional methods.
Since the luminescent component and camouflaging component involve very similar or even the same substances, no camouflage of the substance-based identity of the luminescent component is obtained, since the employment of the camouflaging component increases the total examinable quantity of material of the security feature in the value document to be secured, thereby tending to facilitate rather than impede the analyzability of the security feature or the luminescent component.
Starting out from this prior art, the invention is based on the object of specifying a security feature having a luminescent component and a component camouflaging the luminescent component wherein the analysis of kind and doping of a host lattice employed for the luminescent component is to be prevented or at least substantially impeded. It is intended here that a camouflage of the luminescent component be obtained both with regard to an elemental analysis and with regard to a structural analysis. The identification of the luminescent component is also to be impeded in case the security feature is present in pure form prior to incorporation into value documents or by ashing of authentic value documents, and can then be examined by means of elemental analysis methods such as XRF (X-ray fluorescence analysis) or ICP-AES (inductively coupled plasma optical emission spectrometry) or structural analysis methods such as X-ray powder diffractometry.